Why is it so difficult to have Apple Watch save third party audio files?

The question is from a technical perspective. There are no third party apps that can reliably save audio on the watch. I’ve tried various podcast apps with the feature, but it’s either extremely slow or just doesn’t work. Overcast had the feature through a “workaround” as the developer put it. But then when the new Apple Watch OS came out, the workaround didn’t work any longer so the feature was removed.

What is it that makes this so difficult from a technical perspective? What is it Apple Watch doesn’t support well enough for this to be a feature?